Ultrasonic measurement devices for determining the distance to an object are well known, as described for example in Dumas U.S. Pat. No. 4,254,478 and Makino et al. U.S. Pat. No. 4,581,726. Such devices transmit an ultrasonic wave through a fluid medium such as air or water and receive its reflection back from the object. Based on the speed of the ultrasonic wave through the fluid medium the device may determine the distance to the object very accurately. Such ultrasonic distance measuring devices have a great many applications, including navigation, depth sensors, liquid level sensors and other uses.
However, ultrasonic distance measuring devices are quite sensitive to temperature change in the fluid medium because of the effect that temperature has on the speed of sound through the fluid. Accordingly, many devices have means for correcting the temperature of the fluid. Some devices have been developed which measure the temperature using an ultrasonic transmission through a known distance, such as Shah U.S. Pat. No. 4,541,732. Accordingly, one known means of compensating for the variation in the propagation of ultrasonic waves is by transmitting and receiving an ultrasonic wave travelling through a fixed and known distance. This technique has been employed in commonly owned U.S. Application Ser. No. 07/312,128 filed Feb. 17, 1989, now U.S. Pat. No. 497,197, which is continuation of U.S. Patent Application Ser. No. 06/868,776 filed May 22, 1986, now abandoned. In the commonly owned device the distance measured is between a nuclear density gauge and the surface of a material being compacted. In the device, the distance between the nuclear gauge and the surface is of critical importance, significantly affecting the measurement of the density of the material. However, while measuring the density of hot asphalt, the air above the asphalt may become stratified into layers of varying temperature air. Therefore the distance measuring ultrasonic pulse is traveling through air of a different temperature than the air that the reference measuring ultrasonic pulse travels through. This can cause errors in the distance measurement that would adversely affect the accuracy of the density measurement.
Accordingly, it is an object of the present invention to provide a method and apparatus for accurately measuring the distance and which overcomes the above noted disadvantages and limitations of the prior art.
It is a more particular object of the present invention to provide a method and apparatus for accurately measuring the distance to an object which compensates for homogeneous and nonhomogeneous variations in ambient fluid temperature.